An automobile which obtains driving power from an electric motor (a so-called electric vehicle or hybrid vehicle) includes a high voltage storage battery for driving the electric motor used as the power and a low voltage storage battery for operating auxiliary machinery, such as a light or a radio of the vehicle. A power conversion device which performs power conversion from the high voltage storage battery to the low voltage storage battery or power conversion from the low voltage storage battery to the high voltage storage battery (a DC-DC converter device) is mounted on such a vehicle. It should be noted that the DC-DC converter device is either a device stored in an independent housing or a device stored in the same housing as another device necessary for the vehicle (e.g., an inverter device for driving the electric motor).
Generally, the DC-DC converter device has a circuit which converts from a DC high voltage power supply supplied from a vehicle side to a DC low voltage power supply (or converts from the DC low voltage power supply supplied from the vehicle side to the DC high voltage power supply) (PTL 1). In such a DC-DC converter, the DC high voltage power supply supplied from the vehicle side passes through a power transmission line and is supplied to the DC-DC converter device via a high voltage dedicated connector. Generally, mainly for safety reasons, this high voltage dedicated connector is provided with a connection detection line capable of detecting whether the connector is reliably engaged in terms of a circuit.